Polymer particles have achieved wide use for many purposes including pharmaceutical applications, catalytic applications, carriers for encapsulated materials such as magnetic materials, ceramic carriers for electronic applications, as standards and calibration means for particle study systems, controls for administration of mediciments, filler for chromatographic columns and as research tools for the study of various physical phenomina.
Pigment particles dispersed in an electrical insulating medium have been widely established as important alternatives to dry toners for electrophotography. Often the pigment particles have been adherently bound to the surface of polymer particles and dispersed in a dispersant medium. Problems of poor pigment adherence, flocculation, poor dispersal characteristics, lack of size and morphology uniformity, colloidal instability as well as difficulty in transfering the toner to and bonding same on the image carrier medium.
Accordingly, efforts have been made to provide polymer particle carriers for pigments and polymer-encapsulated pigment particles for the purpose of eliminating or at least reducing the above mentioned difficulties encountered with the above mentioned liquid toners. Liquid toners generally include a dispersion medium, a polymer binder, a steric stabilizer, the pigment and a charge control agent. The dispersion medium commonly employed is an aliphatic isoparafin hydrocarbon such as sold under the trademark ISOPAR by Exxon Corporation. This dispersion medium is electrically insulating, has a resistivity greater than 10.sup.-14 ohm-centimeters, a low freezing temperature, a viscosity of approximately 1.5 cSt at room temperature, a low melting temperature and good electrochemical stability. Steric stabilizers are employed to aid in the dispersal of the particles in the dispersion medium, to stabilize the particle against flocculation and to fix the toner to the paper forming a satisfactory polymer film. Block and graft copolymers have been preferred for steric stabilizers.
The type of pigment particles employed were either organic or inorganic materials, depending upon the color of the image required. The pigment should be embedded within the toner particle or so strongly attached to the surface thereof so that the pigment particle does not become separated from the polymer. Such separation may cause a background image and/or may compromise the photoactivity of the colorant by absorbing incident light during illumination. Another important criterion for the liquid toner is its ability to undergo electrophoresis. This requires use of a charge control agent which controls the magnitude and polarity of the surface charge and thus determines the mobility and electrostatic force of the particle. The proper charge control agent also minimizes the presence of free counter-ions that contribute to the conductivity of the dispersion medium and discharge the electrostatic latent image being developed, i.e. toned.
Key qualities sought for liquid toner particles include control of particle size, distribution and morphology; uniformity of particle size; colloidal stability; reduction in the incidence of impurities; and good encapsulation of the pigment particles within the polymer binder. Also advantage would be gained if the produced particles would be provided in dry form with the retention of uniformity in size and morphology desired, as well as the production of non-pigmented particles as well as the pigment-encapsulated particles with like resultant physical characteristics capable of other, versatile applications. Of considerable importance is achieving control of the surface characteristics of the particles, such as surface roughness and surface area.
Traditional methods to form polymeric particles particularly for dispersal in insulating liquid media for use as liquid toners include emulsion polymerization and dispersion polymerization. These methods as practiced heretofore generally require complex recipes involving selected monomers, emulsifiers, dispersants, initiators, inhibitors, etc. It would be advantageous to simplify the recipes and procedures. Known dispersion polymerization methods provide particles which have very smooth surfaces reducing the number of available charge sites on such surfaces. The importance of providing for control of the surface characteristics of the particles such as increased surface roughness and increased surface area is underscored by increase in the number of sites for charge adherence, improved charge distribution and retention by the particle, better adhesion for adherent components and, in the case of toners, adhesion (bonding) to the surfaces carrying the toner image.
Emulsion polymerization processes have results similar to the dispersion polymerization processes. Another conventional method is to precipitate a polymer from a monomer solution, resulting in sponge formation, said sponge requiring a milling or grinding process to reduce the sponge mass to a useful particle size. The resulting ground sponge results in non-uniform size and morphology. Typical processes involving grinding and/or milling steps to reduce the particles size include U.S. Pat. Nos. 4,842,974, 4,820,605, 4,794,651, 4,758,494, 4,631,244, 4,594,305, 4,526,852, 4,525,446 and 4,306,009.
U.S. Pat. No. 4,306,009 involves the use of a thermoplastic monomer dissolved in a non-polar liquid under temperature conditions required to plasticize and liquify the resin and then cooling the resultant solution with stirring to precipitate particles which still require ball-milling. Non-uniform size distribution results. In addition, the particles are formed with plural fibrous extensions which cause the particles to mat during the toning process, a characteristic resulting in a deposition which require careful control of toning conditions in order to control the thickness of the resulting image as well as resulting in limitation of the type of pigments capable of use therewith. With the production of particles having fibrils or the like, separation of the particles and subsequent drying to provide a dry powder toner which can be dispersed in the selected insulating liquid was not provided, the product being the particles dispersed in the liquid dispersion medium rather than dry powder which can be more easily shipped and stored.
Another method for providing toner particles having a pigment component is taught by U.S. Pat. No. 4,595,646, and provides coating of the pigment particles with a resin component, the pigment having first been treated with a humic acid salt and a humic acid derivative and the resultant particle then being coated with the resin component. The particles are dried and are pulverized to reduce their size. The result often is non-uniform size and morphology, as well as the lack of control of such physical characteristics. The '646 patent employs an aqueous solution of the pigment and the humic acid component to which an organic solvent solution of the resin component is added. Water is removed from the surface of the pigment/humic acid component particles to cover the surface of the said particles with the resin component. Water is removed from the resulting composition and then solvent is removed to result in the dry resin coated pigment particles, the latter being thereafter pulverized. Precipitation of polymer-encapsulated pigment particles is not taught.
Polymerization processes generally have involved copolymerization of two or more monomer components which have required complex procedures and conditions. U.S. Pat. No. 4,081,391 is representative. Again difficulties in controlling the physical characteristics of the resulting particles as to size distribution, surface characteristics and uniformity of size and morphology have been encountered.
U.S. Pat. No. 4,415,645 provides a core of a soft fixing material such as a low molecular weight olefin resin or wax with a hard resin layer containing a pigment surrounding said core. The core particle component is formed by spraying. Uniformity of size, morphology and particle physical characteristics as yet have not been achieved.
In U.S. Pat. Nos. 4,842,975, 4,480,865, 4,618,557 and 4,614,699 dispersion polymerization is employed wherein a first monomer is polymerized to form an insoluble polymer and a second monomer is absorbed onto the surface of the said insoluble polymer and the resultant pair is copolymerized.
Accordingly, there has been a long felt need for a method of preparing finely divided polymer particles, including polymer-encapsulated pigment particles, of smaller and more uniform size that those made by conventional, prior, traditional techniques. Further, it would be of advantage to provide a method forming both polymer particles and polymer-encapsulated particulates, including polymer-encapsulated pigment particles which is simplified over the traditional technique, which is easy to perform, which results in better control of particle size and morphology, which results in less contamination, which is versatile, which enables control of the surface properties, such as surface roughness, surface modifications and surface area of the particles, which can be employed to encapsulate different materials such as pigments, metals, magnetic particles for application to discs or tape, which is capable of encapsulating ceramic materials for electronic applications and which is capable of employment in providing ceramic materials for electronic applications and/or encapsulated animal or plant cells for pharmaceutical applications and which can result in the provision of the toner as a dry powder so that shipping and/or storage costs can be reduced over the liquid dispersed compositions.
A need also has arisen for a method for forming polymer particles wherein such method is sufficiently versatile to enable the preparation of polymer particles of controlled surface area, size and morphology, said particles being free of encapsulated components.
Additionally, a significant need has arisen for the provision of a relatively inexpensive and simplified method for producing mono-dispersed particles for many applications including functioning as standards, as calibration of particle study systems, use as chromatographic column media and as controls for administration of mediciments where such method also would capable of being performed with increased speed over presently available methods.